Virtual endoscopy allows visualization of a scene setup from pre-acquired data. Images are created with a virtual camera interactively manipulated by a user. Imaging techniques currently in use and how the acquired data is visualized are first briefly reviewed below.
At the present time, computed tomography (CT) and magnetic resonance (MR) imaging are the two popular imaging techniques used to capture images of a patient's anatomy, in accordance with known techniques. With CT, different X-ray images taken from a rotational scanner are used to reconstruct a 3D volume. Images of various portions and parts of the internal anatomy will generally appear in an image with different intensities. With MR resonance, relaxation times from excited hydrogen molecules are measured. Using the right parameters, it is possible to image different tissues. Both techniques provide serviceable resolution. In another promising 3D imaging technique, ultrasound is used to create images by localizing sound echoes from an ultrasound emitter. Recent hardware apparatus allows 3D acquisition of data.
It is also possible to interpolate 3D volumes with the composition of 2D ultrasound frames. Acquired data can be visualized in a volume renderer, in a virtual endoscopy, or as single slices. It can be any arbitrary slice, called multi-planar reconstruction (MPR). By manipulating these slices, it is possible to get a 3D sense of the volume. In a volume renderer, the data is directly displayed in 3D. It is possible to rotate around the volume, and to zoom or pan the volume. By rendering certain intensities of the data, different tissues can be revealed. As an additional help to seeing inside, the volume can be cut or intercepted by an arbitrary plane.
In a virtual endoscopy, a virtual camera is located inside the volume. The camera can be moved or rotated and it is possible to change the camera parameters such as its field of view. To define what constitutes a cavity in the volume, an iso-surface value is chosen. Intensities below this value will be considered a cavity, and intensities above it will be rendered as opaque. In the endoscopic view, the boundary seen will therefore be dependent on the iso-surface value.
To render a virtual endoscopic view, two methods can be used: ray casting and iso-surface visualization. The first method casts rays from the virtual camera and detects when the rays hit the volume. At the boundary, the surface normal is used to compute the correct lighting of the current pixel. Each ray will generate a pixel on the screen. The second method extracts an iso-surface from the volume, for instance. with the marching cube algorithm. The resulting mesh is placed in a 3D world and the virtual camera can navigate the mesh. This can be efficiently done with OpenGL, or any other suitable graphic language. Background information can be found in the literature: see, for example, the literature cited below.
The current state of the art visualization remains mostly static. For an extensive review of current endoscopic methods see, for example, Anna Vilanova i Bartroli, “Visualization Techniques for Virtual Endoscopy”, PhD thesis, Technische Universitat Wien, September 2001.
A 4D volume renderer and dynamic MPR are currently available: see, for example, Kostas Anagnostou, Tim J. Atherton, Andrew E. Waterfall, “4D volume Rendering With The Shear Warp Factorisation”, Proceedings of the 2000 IEEE Symposium on Volume Visualization. pp. 129-137, 2000; however, this material does not show a 4D virtual endoscopy system.
Textbooks useful in providing background material helpful to gaining a better understanding of the present invention include, for example, VIRTUAL ENDOSCOPY and RELATED 3D TECHNIQUES by P. Rogalla et al., Springer, Berlin & New York, 2002, FUNDAMENTALS OF IMAGE PROCESSING by Arthur R. Weeks, SPIE Optical Engineering Press & IEEE Press; 1996; IMAGE PROCESSING, ANALYSIS, AND MACHINE VISION, Second Edition, by Milan Sonka et al., PWS Publishing; 1999; and DIGITAL IMAGE PROCESSING, Second Edition, by Rafael C. Gonzalez et al., Prentice Hall; 2002.